Combustion processes which take place at high temperatures cause unacceptably high NO.sub.x emissions, he avoidance or reduction of which is nowadays required by the authorities in certain countries for reasons of environmental protection. This principally concerns industrial combustion plants and also, in particular, gas turbines. Pre-mix burners have therefore been developed for the latter for gas firing. This technology of pre-mix combustion is the most promising method for substantial reduction of NO.sub.x formation with gaseous fuels.
In practice this technology cannot be applied to liquid fuels on account of the short ignition delay time-diesel ignition occurs at high pressure. Other possibilities were need for liquid fuel combustion with low generation of pollutants. Allowing the combustion to take place in two phases appeared to be a promising method. In this so-called two-stage combustion, pre-combustion of the fuel takes place in a pre-combustion chamber under sub-stoichiometric mixing conditions, for instance at an air ratio .lambda.=0.7. With such a strongly sub-stoichiometric combustion, very little NO.sub.x is produced, whereas in an approximately stoichiometric mixture, that is with .lambda. close to 1, NO.sub.x is formed. Likewise, in combustion processes with .lambda.&gt;&gt;1, that is with a large excess of air and a correspondingly cool flame, only a small amount of NO.sub.x is produced.
The reactions involved in the formation of NO.sub.x take place relatively slowly so that it is possible to avoid a high production rate of NO.sub.x, which occurs at .lambda.=1, by very rapid addition of air to the mixture of combustion gases and still unburned fuel flowing from the end of the pre-combustion chamber. Secondary combustion of the over-stoichiometric fuel/air mixture formed under these conditions with .lambda.&gt;&gt;1 then takes place in a second combustion chamber. The desired reduction of NO.sub.x formation by such a two-stage combustion process has been confirmed experimentally, see the article by R. E. Johns "Gas turbine Engines Emissions-Problems, Progress and Future" in the journal "Progr. Energy Combust. Sci.", Vol. IV, 1978, pp. 73-113. However, difficulties occur in the practical application of this concept in that the pre-combustion generates extremely high temperatures with a correspondingly high degree of heating of the walls of the pre-combustion chamber. The usual cooling methods in normal combustion chambers, such as film cooling and convective cooling, are unsuitable for such pre-combustion chambers because under these conditions the cooling air passing into the combustion mixture brings the air ratio close to the stoichiometric region which in turn leads to heavier formation of NO.sub.x which should in fact be reduced by the incomplete pre-combustion.